New research from gravitational wave and neutrino observatories is reshaping our understanding of black holes, stellar evolution, and the nature of dark matter.
A series of recent studies, published across several journals, have analyzed data from gravitational wave observatories and neutrino telescopes. The findings present new insights on the formation of black holes, the existence of predicted stellar phenomena, and potential candidates for dark matter.
The research identifies distinct populations of black holes, provides evidence for a predicted "forbidden gap" in black hole masses, proposes sources for an exceptionally high-energy neutrino, and suggests a method to search for dark matter imprints in gravitational wave signals.
Black Hole Populations and Formation Pathways
Two Distinct Black Hole Populations
A study published in Nature Astronomy analyzed gravitational wave data from the LIGO-Virgo-KAGRA (LVK) collaboration’s GWTC-4 catalog, which includes 153 black hole merger detections. The analysis identified two distinct populations of black holes.
- A lower-mass population, consistent with formation from the collapse of massive stars (supernovae).
- A higher-mass population, with masses above approximately 45 solar masses. These black holes exhibit more rapid spins with random orientations.
Hierarchical Mergers
The spin characteristics of the high-mass population match predictions for black holes formed through hierarchical mergers in dense star clusters. This process involves repeated mergers of smaller black holes to form larger ones.
"The largest black holes detected via gravitational waves are not from direct stellar collapse but from dynamical interactions in star clusters," said lead researcher Fabio Antonini from Cardiff University.
Evidence for the Pair-Instability Mass Gap
Another international study, led by Monash University and published in Nature, analyzed data from the LIGO–Virgo–KAGRA Collaboration. The research identified a specific mass range—above approximately 45 solar masses—where black holes formed directly from stars are rare.
This observed "forbidden gap" is consistent with the theoretical model of a pair-instability supernova. In this model, extremely massive stars reach internal temperatures that cause a reaction reducing internal pressure, leading to a runaway thermonuclear explosion that completely destroys the star and leaves no black hole remnant.
Hui Tong from Monash University, the study's project lead, stated that the findings confirm a mass range where stars do not appear to form black holes directly. The researchers noted that any black holes observed within this mass range likely originate from the merger of smaller black holes, rather than from direct stellar collapse.
Key collaborators included Professor Maya Fishbach from the University of Toronto and Professor Eric Thrane from Monash University. The existence of this mass gap presents new questions for astrophysics regarding stellar evolution models and the frequency of these explosions.
The Ultra-High-Energy Neutrino: KM3-230213A
Detection of an Energetic Neutrino
In 2023, the Cubic Kilometer Neutrino Telescope (KM3NeT), located off the coast of Sicily, detected a neutrino designated KM3-230213A. The particle was measured to have an energy of approximately 220 PeV, making it significantly more energetic than any previously observed by the IceCube observatory in Antarctica. This energy level is approximately 100,000 times greater than particles produced by the Large Hadron Collider.
Proposed Origins
Multiple hypotheses have been proposed to explain the origin of this high-energy neutrino.
1. Exploding Primordial Black Holes:
A research team from the University of Massachusetts Amherst, in a study published in Physical Review Letters, proposed that the neutrino could have originated from the explosion of a hypothesised "quasi-extremal primordial black hole" (PBH). Primordial black holes are theoretical objects thought to have formed from density spikes shortly after the Big Bang. The hypothesis relies on Stephen Hawking’s theory of Hawking Radiation.
The team suggested that PBHs possessing a "dark charge"—a hypothetical copy of the electromagnetic force—could explain the event. This hypothesis addresses a key discrepancy: IceCube did not detect an event of comparable energy. The researchers propose that quasi-extremal PBHs with a dark charge could account for the single detection, whereas standard PBHs would have produced more frequent high-energy neutrinos.
If a substantial population of such PBHs exists, the model may also provide an explanation for dark matter.
2. Blazars:
A separate paper published in the Journal of Cosmology and Astroparticle Physics (JCAP) by the KM3NeT collaboration proposed that the neutrino’s source could be a population of blazars. Blazars are active galactic nuclei that host supermassive black holes and emit plasma jets directed toward Earth.
The researchers simulated a blazar population and calculated the expected neutrino and gamma-ray fluxes. Their model accounted for the rarity of ultra-high-energy events in existing datasets and was consistent with observations from KM3NeT, IceCube, and the Fermi Gamma-ray Space Telescope. The absence of an electromagnetic counterpart to the neutrino event suggests a diffuse background source rather than a single catastrophic event.
At the time of detection, the KM3NeT detector was operating with approximately 10% of its final planned volume. Validation of either hypothesis requires further data and analysis.
A Method to Detect Dark Matter Imprints in Gravitational Waves
Researchers at MIT and European institutions developed a method to predict gravitational wave signals from black hole mergers occurring in environments with high-density dark matter. The method was applied to 28 gravitational wave signals from the LVK observatories' first three observing runs.
Of the signals analyzed, 27 were consistent with mergers occurring in a vacuum. One signal, GW190728, showed a preference for the dark matter model, but with low statistical significance.
The study, published in Physical Review Letters, provides a new tool for screening gravitational wave data for potential signatures of dark matter.
Potential Gravitational-Wave Signal of a Primordial Black Hole
On November 12, 2025, the LIGO observatories detected a gravitational wave signal, designated S251112cm. The signal indicated a merger involving an object with a mass less than the sun—a characteristic expected for primordial black holes.
Researchers at the University of Miami, including Nico Cappelluti and Alberto Magaraggia, published a study in the Astrophysical Journal suggesting the signal is consistent with a primordial black hole merger. Their quantitative framework calculates that the rarity of such sub-solar black hole mergers aligns with the predicted abundance of primordial black holes, and that these objects could account for a meaningful share of dark matter.
The signal's false alarm rate is estimated at approximately once every four years.
The researchers noted that this single detection does not confirm the existence of primordial black holes and that final parameter analysis from the LIGO collaboration was pending. Future upgrades to LIGO and the planned launch of the European Space Agency's Laser Interferometer Space Antenna (LISA) in 2035 are expected to provide more sensitive searches for sub-solar merger events.